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one day
all day (long); the whole day; from morning till night
first day of the month
「一日」 has other readings with different meanings.
一日中ずっと雨だった。
It rained all day long.
一日中働きましたか。
Did you work all day?
一日に三回薬を飲んでいます。
I take medicine three times a day.